The invention relates to a method of determining characteristic rheological quantities of viscoelastic materials, in particular of rubber and rubber mixtures.
More and more, manufacturers are having to observe increasing standards in terms of quality. In addition, early predictions about processing behaviour are useful for the manufacture of products. The development of new products is simplified by characterisation by rheological characteristic quantities. Methods of examination have therefore been under development for a long time but they only meet these requirements to a limited extent.
The parallel plate compression plastimeter is a known method for determining plasticity according to Baader, which was known under the former DIN 53 514 standard.
In this method, a cylindrical body (.phi. 10 mm, h=10 mm) is drilled from a so-called skin and subjected to a load on its face after storage at a temperature of 80.degree. C. The force (deformation hardness) which compresses a virgin sample to a height of 4 mm in 30 seconds is sought by repeated testing as a measured value of the viscous behaviour of the material. The expansion of the material, which is designated as deformation elasticity and which can be considered as a measured value of the elastic behaviour of the material, is measured 30 seconds after unloading.
The investigation supplies only two measured values which are not sufficient to characterise the material, since, on the one hand, the stress conditions vary greatly in practice and, on the other hand, the properties of the material can change during processing and statements cannot be made on either point by this examination. Moreover, the search for the corresponding force is time consuming and costly in material, irrespective of the risk of error.
A mooney rotating plastometer in which a corrugated disc rotates in a cylindrical chamber filled with material to be tested, is also used. The viscosity counteracting the rotation of the disc produces a torque, which can be taken as a measure of viscosity. A characteristic quantity can then be determined for the elasticity by means of the back-twist angle of the rotor on uncoupling the drive.
Considerable variations occur in the measurements, particularly as a result of non-stationary, non-homogeneous temperature fields in the sample and varying adhesion to the walls. In particular, they cannot be reproduced sufficiently from laboratory to laboratory.
A mechanical spectrometer is also known, with which the sample is subjected to shearing deformations. The reactions of the sample are then conveyed to a measuring unit by a transformer which measures the forces in directions which are perpendicular to each other. The device is suitable only for research institutes, owing to its complicated structure.
The two last-mentioned measuring devices have the disadvantage that the determination of the dependence of the viscoelastic characteristic quantities on the deformation rate is very time consuming and costly in material, and also materials having the same flow and temperature pre-history have to be used in each case in order to eliminate additional influences.